Detect drag and dropped files from python?
No, it's not possible. In Blender's C code, I found references to "dropboxes" and code to deal with them in different area types, but the process is completely inaccessible through Python (no variables to check, nor callbacks to register).

There is bpy.ops.object.drop_named_image, that looks like a modal operator. It supposedly adds a dropped image as an empty, but does nothing when executed and crashes Blender when invoked, so I think it's an internal operator just like bpy.ops.mesh.drop_named_image. So there's no directly-callable Python operator for drag-and-drop either.

Besides, Blender doesn't support URI-formatted filepath.